Suspensions of coal and coal by-products in fuel oil have been proposed as a fuel for blast furnaces, boilers and other fuel burning devices, but because of the explosion hazard in grinding dry coal and the difficulty in handling the suspensions, no practical system has yet been devised for the large scale use of this combination fuel.
U.S. Pat. No. 2,118,477, issued May 24, 1938 to Roberts teaches the combination of colloidal coal particles and oil to form a suspension for use as a composite fuel. The difficulty and expense in grinding coal to colloidal fineness, such that it passes through a 300 to 1,000 mesh sieve has prevented the composition fuel disclosed by Roberts from being used in present day furnaces.
U.S. Pat. No. 2,231,513, issued Feb. 11, 1948 to Stillman teaches a stable suspension of coal and oil made by pre-grinding coal particles until they pass through a 100-mesh screen and thereafter introducing the pre-ground particles into oil and subjecting the combination to a further grinding until the particle size is reduced to about 5 microns. All of the coal particles are small enough to pass through a 325-mesh screen, and at least 50% of the particles are under 10 microns in size, which is sufficient for the particles to produce the phenomenum known as the "Brownian Movement." The Brownian Movement of the particles is sufficient to maintain all of the coal particles in stable suspension.
Other patents, such as the Plauson et al. U.S. Pat. No. 1,647,471, issued Nov. 1, 1927, teach the combination of pre-ground carbonaceous material with an oil. The Plauson et al. patent teaches pre-grinding the carbonaceous material to a powder which passes through a 125 to 250 mesh screen, and thereafter, forming an emulsion with the oil by the addition of from about 1 to about 3% of a soap solution. The Plauson et al. patent has an additional disadvantage besides the small particle size of the carbonaceous material as the suspension or emulsion is produced in small batches using a cross hammer mill rotating at 325 feet per minute.
All of the above referred to patents teach combination liquid and solid fuels, which are difficult or dangerous to produce, resulting in the fact that combination solid and liquid fuels are presently not in use. Particularly, any method which requires that the coal be ground in the dry state presents a potential explosive hazard and should be avoided. Processes in which coal is colloidal in size require grinding times in excess of 5 or 6 hours, making the process uneconomical. These and other disadvantages are obviated by the present invention.